Looks like it had been noted here before that the wimfltr driver cannot be used on windows 7 sp1 wims due to a format change by microsoft.
I tried with multiPE under win7 Sp1 x86 host and built a PE from Win7 SP1 source.
I found an issue due to Win7 SP1. The PE reboots when the desktop appears.
When writing into loaded registry hives, the Win7 SP1 host generates some magic log files.
When I delete these magic files before creating the ISO, everything works well.
Win7 SP1 x86 host.
Win7 SP1 or SP0 source CD
Imagex & co from Vista WAIK Sp1 (6.0.6001) with installed wimfltr.sys, mounts within seconds.
Before creating the ISO:
PE boots well and runs fine.
And now the bad news:
Including MMC brings an error in the PE, that compmgmt.mmc is corrupt. It has really only 2 kb size.
Until no new fixes, work arounds, the Vista imagex / wimfltr method can be used ONLY on hosts BELOW win7sp1.
I'll change in multiPE accordingly.